Cathode ray tube



April 1938. M. VON ARDENNE CATHODE RAY TUBE Filed May 24, 1934 INVENTORVon fir denne anfred HTTORHEY Patented Apr. 26, 1938 UNITED STATESPATENT OFFICE CATHODE RAY TUBE Application May 24, 1934, Serial No.727,207 In Germany June 10, 1933 13 Claims.

My invention relates to electron discharge devices and more particularlyto cathode ray tubes as used for oscillographs and in televisiontransmission systems or similar electronic devices using a concentratedelectron beam.

The general form of cathode ray tube as known in the art and used foroscillographs and in the cathode ray television systems, containsessentially the three parts: a source for producing a thin stream orpencil of electrons traveling at very high velocity, a fluorescenttarget or luminous screen for the electrons to strike against to producea luminescent spot, and some mechanism for deflecting the path of theelectron pencil in any direction and for varying the intensity of theelectron beam to produce any desired pattern or image upon thefluorescent screen.

While my invention is in particular useful for cathode ray tubesincluding a gaseous atmosphere for focusing the electron beam, it isunderstood that the novel features of the invention as will be set forthare equally applicable to tubes operating with a partial or a highvacuum.

., Cathode ray tubes of known type utilizing a gas for focusing theelectron beam are characterized by their very great simplicity of bothconstruction and operation.

One of the major disadvantages of cathode ray tubes of this type is thefact that the brightness of the luminous spot on the fluorescent screenis insuflicient for many uses, such as for direct projection of atelevision image on a screen similar as in a moving picture or lanternprojector.

It has not been possible to increase the anode voltage beyond severalthousand volts for increasing the brightness of the luminous spot on thescreen due to undesired interference with the concentrating action ofthe gas as the anode 40 voltage increases, and it has furthermore notbeen possible to materially increase the degree of luminescence bymeasures and improvements applied to the source of the cathode ray.

Accordingly, it is one of the main objects of my invention to providemeans in connection with a cathode ray tube for substantially increasingthe degree of brightness of the fluorescent spot on the luminous screenas compared with tubes of this type heretofore known. in the art.

50 There are several reasons for the limited luminosity of thefluorescent spot in cathode ray tubes hitherto known in which atranslucent fluorescent screen is applied to the inside wall of the tubewhereby the viewing direction is against the outer 55 or rear side ofthe screen.

One disadvantage of a screen of this type is the fact that theluminescent energy produced by the impact of the electrons isdistributed over two sides of the screen; that is, the front side andthe rear side, resulting in substantial reduction 5 of the usefulluminescence, as is obvious.

Accordingly, it is another object of my invention to provide a luminousscreen for use in cathode ray tubes in which the luminous energy isapplied to the front side of the screen only im- 0 pinged upon by theelectron beam for increasing the degree of luminescence obtained.

Another disadvantage of screens used in tubes heretofore known is due tothe fact that the screen has to be very thin to produce sufficient lumi-15 nosity at the rear side which in turn entails a reduction of theluminosity by absorption by the screen itself.

Accordingly, a further object of my invention consists in the provisionof a fluorescent screen for 20 use in cathode ray tubes in which thethickness of the screen is not limited, enabling the use of a screen ofincreased thickness as compared to hitherto known tubes for increasingthe degree of fluorescence obtained.

Another disadvantage inherent in cathode ray tubes of known constructionis the fact that a large amount of light passing the glass wall of thetube to which the screen is applied, is lost by total reflection fromthe outer wall of the tube with the added disadvantage of disturbinghalo efiects being produced which in turn greatly impair the detail ordefinition of the pattern or image recorded on the screen.

Accordingly it is another object of my invention to provide afluorescent screen for use in cathode ray tubes in which losses by totalreflection and a blurring of the picture by halo efiects due to totalreflection from theouter wall of the base to which the screen is appliedis substantial- 1y eliminated.

Still a further disadvantage of luminous screens used in cathode raytubes heretofore known is the fact that on accountof the bad heatconductivity of the glass wall upon which the screen is applied, 5 theparticles of the screen impacted by the impinging electron pencil becameexcessively heated, resulting in impairment of the luminescence, causingin turn a decrease of brightness of the luminous spot and a blurring orloss of detail of the pattern or image produced.

Accordingly it is another object of my invention to provide a luminousscreen for use in cathode ray tubes in which heat produced by the impactof the electron pencil upon the screen is angles and furthermore theirregularities of the glass of the tube result in great opticaldistortions, as pointed out.

I overcome these disadvantages in accordance with a further feature ofthe invention by inclining the plane of the screen relative to the axisof the tube preferably at an angle of 45 with the axis of one of the twopairs of deflecting plates. In this manner, it is possible to View thescreen at right angle to its surface or to reproduce or project thepattern or image by means of an optical system having its axis arrangedat right angle to the screen surface. It is furthermore possible by suchan arrangement to mount a high speed lens system of short focal lengthat a minimum distance from the screen enabling the use of inexpensivelenses of high power or relative aperture as the price of a lens ofgiven speed or relative aperture decreases, the shorter the focallength, as is well known. For this purpose, I provide a spherical endportion of the tube in which the luminous screen is arranged which hasthe further advantage that the glass wall may be manufactured free fromany defects or surface irregularities, resulting in an elimination ofoptical distortions.

Referring more particularly to Figure 1, I have shown a cathode ray tubeconstruction according to the invention comprising a glass bulb Iconsisting of a cylindrical neck portion housing the source producingthe electron ray and the deflecting and controlling mechanism and aspherical end portion enclosing the luminous screen 1 arranged at anangle of 45 to the axis of the tube. The tube is mounted upon a base 2and provided with a press 3 acting as a support for the cathode and thewell known cylindrical or concentration electrode 5. The cathode 4 maybe either a directly heated'cathode as shown,

or it may be an indirectly heated cathode Well a known in the art. Thecathode terminals are connected to prongs 4 and the concentrationcylinder 5 is connected to a prong terminal 5 mounted at the bottom ofthe base 2 for connection to the proper operating potentials. I havefurthermore shown at 6 an anode in the form of a disc with a centralaperture for passing the electron pencil produced by the cathode 4 andthe concentration cylinder 5. Numerals 8, 8 and 9, 9' represent theusual pairs of deflecting plates arranged at right angle to each otherfor both horizontal and vertical deflection of the cathode ray. I havefurthermore shown at II] and II terminals for applying the deflecting orsweep potential to the plates 8 and 8', respectively, and imilarterminals may be provided for the plates 9 and 9' as is understood.

The luminous screen 1 consists of a base plate such as a metal plateupon which there is applied a coating of luminous material I, such as ofzinc silicate or the like well known in the art. I have furthermoreshown the screen I connected to an outside terminal l2 through lead I!for applying a potential to a screen equal to the anode potential, asshown, to prevent disturbing back current from the screen to the anodeelectrode. I have furthermore shown a constricted portion of the tubeclose to the spherical end portion, enabling further the close mountingof an inexpensive optical lens system of short focal length, as will beseen more clearly from Figure 4. The diameter of the constricted portionis limited by the dimension of the screen 1 and should be such as toallow a maximum sweeping angle of the electron pencil to cover theentire surface of the screen 1. In order to prevent the effect ofdisturbing wall charges, I have shown a further electrode l3 applied tothe outside Wall of the constricted portion maintained at anodepotential by the connection, as shown.

The inclined mounting of the screen involves various added distortionsof the luminous spot or the pattern or image produced on the screen I.These distortions increase as the length of the tube or of the electronray, respectively, decreases, a procedure which I have foundadvantageous, as will be pointed out hereinafter.

In general, especially in the case of television, it is required thatthe sensitivity of deflection is substantially equal for both pairs ofdeflecting plates. By the provision of an inclined screen and assumingthat the distance between the deflecting plates in both deflectingsystems is the same, it is seen that the deflecting plate systemarranged close to the anode electrode which is furthermore at a greaterdistance from the screen than the other plate system, has a considerablyincreasedsensitivity of deflection. I compensate for this difference ofsensitivity by making the sensitivity of the plates arranged close tothe anode less than the sensitivity of the plates arranged close to thescreen. This may be obtained, for instance, as shown by Figures 1 and 2,by varying the distance between the deflecting plates; that is, by usinga larger distance between the plates 8 and 8' arranged close to theanode 6 as compared with the distance between the plates 9 and 9'.

A further distortion is produced by the deflection caused by plates 9and 9' in a vertical direction due to the inclined position of thescreen I. This has the effect, as is seen, that the portions of theimage or pattern at the part of the screen which is at closer distanceto the deflecting system are concentrated while the portions of theimage or pattern at larger distance from the deflecting system areexpanded. Thus in the case of equal deflecting or sweep voltages appliedto both pairs of deflecting plates, which under normal conditions wouldresult in a luminous surface of substantially square shape, thedistortion produced due to the inclination of the screen by thedeflecting plates 9 and 9' will result in a surface of trapezoidal shapeas compared to a square when no distortion would be present. I avoidthis distortion in accordance with my invention by arranging plates 9and 9 at an angle such as shown in Figure 3 in such a manner that withinthe range of shorter length of the cathode ray the sensitivity ofdeflection is increased on account of the greater electric fieldstrength between the plates while for the range of longer length of thecathode ray the sensitivity of deflection is less, due to the decreasedfield on account of the larger distance between the plates 9 and 9'- Afurther effect produced by the inclined position of the screen 1 resultsin a distortion of the shape of the luminous spot produced by theimpinging electron pencil. Thus, it is seen that if the cross-section ofthe pencil under normal conditions is substantially circular producing acircular spot on the screen, in the case of an inclined screen as shownthe circular spot will be distorted into an oval or elipsoidal shape. Iovercame this distortion in accordance with a further feature of theinvention by providing a cathode producing a pencil of elipsoidalcross-section, such as by making the emitting surface of the cathode ofoval shape. The same effect may be obtained by using a loop-shapedfilament which, as is known, produces'an oval electron stream. Thus,

1 itis only possible to properly orientate'the stream stood.

clination of the screen 1 in a tube as described is r produced by thecathode so as to compensate the oval distortion to obtain asubstantially circular .spot'on the inclined screen. For'this purposethe large axisof the oval stream should be in a direction of thedeflecting plate .system'mounted close to the fluorescent screen ,(9 and9), as is under- Still a further disadvantage caused'b'y. the indue to,a' decreaserof definition of theluminous spotzat' certain portionsonthe screen; that is, at

those parts on the screen which are at a fartherdistancefromuthecathodethan the remaining parts. 'I'his'resultsin a substantiallack'of-de tail or "definition especially'in thecase of televi: sionimages at those portions of a pattern or picture lying atthe'outerportionsof the screen; that is, for the larger deflecting angles.

I avoid the aforementioned defect and disad- V vantageof an inclinedscreen by the provision of means for applying different potentials. tothe ance material such "as shown at I 1 in Figure 5,

. like.

various points on the luminous screen in sucha r manner as tosubstantially compensate .for' the varying length of the impingingelectron pencil individual elementary areas on; the

upon .the screen.

In accordance with one embodiment of the invention, I provide' a'screenconsisting of resistor 'alternatively'I may use a base plate upon whicha coating of resistance material is applied as by means of any wellknown method, such as by cathodefsputtering or precipitation and. the

nected to metallic connecting elements or electrodes l8 and I8" mountedalong the upper and lower 'edge of the screen I! and in contact with theresistance :surface I! coated'upon a base metal or itself forming thesupport for the 1u.

minous'sc'reen; This is shown iniurther detail byflFigure' 6. In' thismanner a gradual voltage increase from the upper to thelower edge on thescreen is obtained which is added to a normal positive voltage 20applied to the screen as shown in Figure 5. By properly choosing thevoltage l9 it'is seen that by an arrangement of this sort a graduallyincreasing potentialis'applied to the' 7 screen with increasing lengthof the impinging electron pencil, thus' compensating and eliminate; Jing the difference. in detail or'definition .of' the "luminous spot fordifferent ideflecting angles,fas

the fluorescent screen; that is in other words, .by

adjusting the conductivity at different zones on the screen, anydeviations or distorting efiects other than those described due to anycause may be'compensated in this manner, or alternatively anymodification of the picture or pattern or its definition may be adjustedas desired. Thus for instance, it is possible to produce additionalacceleration of the electron pencil between the I have furthermore showna battery l9 con-,

screen and a preceding accelerating electrode, especiallyfor largedeflecting angles in which case celeration of the-electron pencil tosecure a desired luminosity ordegree of definition, as may be desired,of the fluorescent spot'produced by the electron impact. I

the screen itself may act as a means for final ac- In all theaforementioned cases where a resistance material is used in connectionwith the luminescent screen, care should be taken that the resistance ofthe support for the screen between the connecting'electrodes issufiiciently low so as to prevent any substantial potential drop due tocurrentflow caused by the impinging electron pencil and causingundesired. modification and distortion of the electric field. I havefound that in the case of tubes operating with compar atively.lowvoltages, the voltage drop produced by the ray current along the supportof the 'lumi nescent screen shouldbe less than 10 volts. As with theusual tubes the intensity of the ray cur rent is about 1O "amperes, thisvalue would coraprecipitating method, as known in the art.

By using atub e as described hereinbefore with the" additional means forcompensating the various, distortions and errors introduced by theinclinationof the screen, I have been able to secure records of patternsor images on the fluores cent screen absolutely free from anydistortion;

When an optical lenssystem is desired, such frespond to a resistance ofabout 100,000 ohms whichcan easily be obtained by a sputtering or as forreproduction or 'projection'of the screen pattern or image, the size andcost of'the lens will be the less, the smaller the size of thefluorescent screen, as is obvious. .However, a de-- crease of thedimensions of the fluorescent screen is only possible if the diameter ofthe luminous spot can be decreased at the same time. While in. tubes asheretor'orc known in the art in which the luminous screen is directlyapplied to the inside wall of the tube, the diameter of the screen isfrom 12 to 20 centimeters,

it has been possible by using. a construction according to my inventionto decrease'the diameter of the luminous screen to 7.5 centimeters orless. Despite this small dimension'of the flu crescent screen, I havebeen able to secure an exceedingly sharp definition or detailof the pic-'ture by decreasing the length of the ray or of the tubeyrespectively. Ihave found that with decreasing length of theray, especially in the caseof gas filled cathode ray tubes, the sharpness ofthe luminous" spotincreases considerably. As is known, the electron-optical representation of the cathode in the tubesas known in'the art takes placewith considerableenlargement depending, as pointed out, to a largeextent upon the length of'the ray. Investigations have furthermore'shownthat a decrease of the tube' is advantageous not only on account of thedecrease of the diameter of the luminous. spot but also'on account ofthe increase of the current at the top of the ray, resulting in aconsiderable increase of the luminescence produced by impact of the rayupon the luminous screen.

I have .found by measurements that in the usual cathode ray tubesprovided with a gaseous filling, the ray current at the top is onlyabout 20% of the entire emission current, the rest being lost by strayelectrons leaving the ray in a lateral direction.

ably decreased and the current intensity at the end or the recordingplaceis increased by 50% By decreasing the 'ray' length, the loss bystray electrons is considerand more of the total emission current. Thisnot only results in an increase of brightness to about the double value,but simultaneously is accompanied with a decrease of the interfering anddisturbing side luminescence caused by the stray electrons to about halfthe value as compared to cathode ray tubes of standard design.

By means of cathode ray tubes constructed in this manner, images ofexceedingly sharp contrast are obtained due also to the fact that theabove-mentioned disturbing halo effects have been eliminated. It isfurthermore advisable, as pointed out, to connect the metal screen withthe anode to prevent a back flow of the electrons through the gaseousspace in the tube, thus eliminating the number of disturbances caused bysuch back currents.

I have furthermore found it advisable, as pointed out, to prevent wallcharges at the constricted portion of the tube by providing an outermetallic coating also connected to the anode or ground, respectively, asshown.

Referring more particularly to Figure 4, I have shown a cathode ray tubeas illustrated by the previous figures, mounted in a housing 14 upon adisplaceable support H for adjusting the tube relative to an opticalsystem as indicated at l5, such as a projecting apparatus for directprojection of the screen image similar to a lantern slide or movingpicture projector. The tube is mounted in an inclined position as shownand the objective system l5 arranged with its axis at right angle to thesurface on the screen I. I have furthermore shown at IS an opening orwindow for controlling and observing the operation, especially thecathode heating of the tube.

If the image produced by the optical system i5 is reversed as in thecase of an ordinary projector, this may be compensated by optical orelectrical means so that a final upright and right-sided image isobtained on the projection screen. Thus for instance, the image may bereversed by the use of prisms or mirrors as is well known, oralternatively, in accordance with a further feature of my invention, theorientation of the image may be controlled by properly choosing thepolarity of the deflecting voltages applied to the deflecting plates 8,8 and 9, 9', respectively. Thus, in the case of optical projectionwhereby the image is reversed in the projector, the polarity of thesweep voltage moving the beam in the vertical direction should be suchthat the picture is being scanned from the bottom to the top so as toobtain a reversed picture on the luminous screen, which will then appearas an upright picture on the projection screen. Similarly, if therelationship of the sides of the picture is reversed this may becompensated by proper choice of the polarity of the deflecting sweepvoltage moving the beam across the image, as is understood.Alternatively, the tube may be mounted in an upside-down position toobtain a reversed picture on the luminous screen which will appear as anupright picture on the projection screen. According to a furtherfeature, I may provide reversing switches for reversing the polarity ofthe sweep voltages in any desired manner, such as for either directobservation of the picture through a magnifying lens or for projectionby means of a projection lens, as described in more detail in mycopending application entitled Television system, filed June 13, 1934,and bearing the Serial No. 730,408.

By using a tube and an arrangement as described, operating with an anodevoltage of the order of 4000 volts and a gaseous filling consisting of ahydrogen gas, I was able to obtain a bright screen picture enabling adirect projection upon a surface of 2 x 2 meters in a darkened space. Bythe same apparatus, oscillographic records were possible with arecording speed up to- 20 kilometers per second using a high speed lenssystem.

I claim:

l. A cathode ray tube for recording purposes comprising an envelope;means for producing a concentrated, electron beam therein; means fordeflecting said beam in two mutually perpendicular directions; an opaquescreen impinged by the electrons to produce a light spot thereon, saidscreen being mounted at an angle to the line of direction of said beam;and electrostatic means for compensating for recording distortionsproduced by the inclination of saidscreen with respect to a viewingdirection at right angle to said screen. 2. A cathode ray tubecomprising an envelope; means for producing a concentrated electron beamtherein; two pairs of deflecting plates arranged at right angle to eachother for deflecting said beam in conjugate rectangular coordinates; anopaque screen covered with luminescent material for the electrons tostrike against mounted with one coordinate at right angle and with theother coordinate inclined tothe line of direction of said beam; andmeans for compensating image distortions produced by the inclination ofsaid screen.

3. In a cathode ray tube as claimed in claim 2 in which the portion ofsaid envelope enclosing said screen is of substantial spherical shape.

4. A cathode ray tube as claimed in claim 2 in which one pair of saiddeflecting plates is arranged with its'axls parallel toand at closerdistance from said screen and said other pair of deflecting plates isarranged at a further distance from said screen with its axis at anangle of 45 with said screen, and in which the distance between theplates of said latter deflecting system is greater as compared to thedistance between the deflecting plates of said former system. 5. In acathode ray tube as claimed in claim 2 including said screen beingadapted to have biasing potentials applied thereto to secure apredetermined potential distribution of different zones of the screenarea for compensating the effect of varying length of the cathode raydue to the inclination of said screen.

6. In a cathode ray tube as claimed in claim 2 including means forvarying the cross-sectional shape of said cathode ray for compensatingdistortions of the shape of the luminous spot caused by the inclinationof said screen.

'7. In a cathode ray tube as claimed in claim 2 including means tosecure an oval cross-sectional shape of said electron beam forcompensating distortions of the luminous spot on said screen caused bythe inclination of said screen.

8. In a cathode ray tube as claimed in claim 2 in which said luminousscreen is coated upon a base of resistance material, and adapted to have9. Acathode ray tube comprising an envelope,

means for producing a beam therein, a luminous screen in thepath of saidbeam and at an angle thereto, said screen comprising an opaque basemirrored upon the side facing said beam, and the concentrated electronbeam therein, means {or luminous material positioned on said mirroredface of the base. 7 7 10, A cathode ray tube comprising an envelope,

'means forproducing a beam therein, a luminous screen in the path ofsaid beam, said screen comprising a'highly polished opaque aluminumbase;

facing saidbeam and having the luminous mate rialpositioned on saidpolished face.

11. A cathode ray/tube for recording purposes,

comprising an envelope, meansfor producing a deflecting said beam in twomutually perpendicular directions, an opaque screen covered withluminescent material for the electrons, to strike against to produce alight spotthereon, means to support the screen at an angle to thedirection of said beam, said screen being adapted to have electricbiasing potentials applied theretorto prevent I the flow of back currentfrom the screen.

12; A cathode ray tube for recording purposes,

comprising an envelope/means for producing a concentrated electron beamtherein, electrostatic means for deflecting said beam in 'two mutuallyperpendicular directions, an opaque screen covered with luminescentmaterial for the electrons to strike against to produce a light spotthereon, means to support the screen'at an angle to the direction ofsaid beam, said screen being adapted to have electric biasing potentialsapplied thereto to prevent the flow of back current from the screen.

'13. A cathode ray tube comprising an envelope, means for producing aconcentrated electron beam therein, means for deflecting said beam inmutually perpendicular directions, a target eleck trode, means forsupporting the target electrode at an angle to the line of direction ofthe electron beam, and means to develop a potential difference betweenthe beam producing means and the target electrode, said potentialdifierence being of a magnitude progressively changing in accordancewith the distance between the elemental areas or the target and the beamproducing means.

- MANFRED VQN V ARDENNE.

